1. Field of the Invention
The present invention relates to an image forming system for transferring an image formed on an image bearing member onto a transfer sheet carried by a transfer sheet bearing member, and more particularly it relates to an image forming system suitable to be used with color electrophotographic equipment of electrophotographic type of electrostatic type.
2. Related Background Art
In the past, in color electrophotographic equipment, an electrophotographic photosensitive drum acting as an image bearing member is rotatably supported and a toner forming means forming a toner image on the drum is arranged around the drum.
More particularly, the photosensitive drum is uniformly charged by a primary charger and a light image corresponding to image information is illuminated on the photosensitive drum by an exposure means comprising a laser scanner and the like, thus forming an electrostatic latent image on the photosensitive drum. Then, the electrostatic latent image is visualized as a toner image, for example, by a shiftable developing means.
The shiftable developing means comprises four developing devices containing magenta color developer, cyan color developer, yellow color developer and black color developer, respectively, and a guide for holding these four developing devices and shiftable in a horizontal direction. The shiftable developing means permits movement of a desired developing device to a position facing an outer surface of the photosensitive drum.
The various color toner images formed on the photosensitive drum are successively transferred and superimposed on a transfer sheet carried and conveyed by a rotating transfer drum acting as a transfer sheet bearing member.
The photosensitive drum is supported on a rotary shaft so that it is rotated by driving the rotary shaft by a driven motor. The photosensitive drum is provided at its axial end with a flange having a gear which is meshed with a gear integrally formed with the transfer drum in the vicinity of a surface of the latter. Thus, when the photosensitive drum is rotated, the transfer drum is also rotatingly driven.
When such a driving system is used, in order to prevent a so-called "discrepancy in color" in the superimposing transfer of the toner images, techniques wherein an outer diameter of the transfer drum is greater than that of the photosensitive drum by an integral number times have been proposed, as disclosed in the Japanese Patent Laid-Open Nos. 49-113635 and 61-83557. Further, a technique wherein the photosensitive drum and the transfer drum are driven by independent driven motors has also been proposed.
Such discrepancy in color occurs due to the discrepancy in rotational axes and/or in the circularity of the photosensitive drum and/or transfer drum, i.e., a distance between respective points on the surface of the photosensitive drum and the rotational axis thereof or a distance between respective points on the surface of the transfer drum and the rotational axis thereof is not constant. That is to say, if peripheral speeds are different at any plural points on the outer surface of the photosensitive drum or the transfer drum, the discrepancy in the different color images will occur.
Accordingly, in conventional color electrophotographic equipment, when a driving force is transmitted from the gear on the flange of the photosensitive drum to the transfer drum there occurs the limitation in that the outer diameter of the transfer drum must be greater, by an integral number of times, than that of the photosensitive drum, or the high accurate circularity and coaxiality of a pitch circle of the flange gear is required.
For example, if the pitch circle of the flange gear deviates from circularity by a, an amount of deviation or discrepancy on the peripheral surface of the transfer drum will be as follows: ##EQU1## Where, r is a radius of the photosensitive drum (r&gt;&gt;a), .phi. is a phase difference of the photosensitive drum (due to the difference in diameter between the transfer drum and the photosensitive drum) caused when the transfer drum is rotated by one revolution.
The amount of discrepancy becomes maximum when the phase difference is .pi. (.phi.=.pi.), for example, which corresponds to the fact that a ratio between the diameter of the transfer drum and that of the photosensitive drum is 3:2. In this case, the maximum amount of discrepancy becomes 2a. Thus, to limit the discrepancy in color due to the noncoaxiality within 50 .mu.m, high accurate coaxiality within a range of 25 .mu.m or less is required. Further, when the photosensitive drum and the transfer drum are driven independently, two or more motors are required, thus leading to a cost increase.